The silicone rubber foam is well known as a material excellent in weather resistance, electric property, compression set and so on. The silicone rubber foam can be basically obtained by compounding an organic foaming agent such as azobisisobutyronitrile (AIBN), azodicarbonamide (ADCA), dinitrosopentamethylenetetramine (DPT) or the like and a curing agent into a thermosetting millable silicone rubber composition, and then foaming and curing the resultant by heating. In order to obtain a silicone rubber foam having excellent characteristics, it is necessary to balance the foaming rate (decomposition rate) of the organic foaming agent and the curing rate of the silicone rubber composition.
More specifically, if the decomposition rate of the organic foaming agent is lower than the curing rate of the silicone rubber composition, cells become fine but the foaming ratio becomes low, whereas if the curing rate of the silicone rubber composition is lower than the decomposition rate of the organic foaming agent, gas generated by decomposition of the organic foaming agent cannot be sufficiently captured, resulting in a low foaming ratio and an uneven cell structure.
In addition, since the organic foaming agent has a high decomposition rate and rapidly generates gas, it is necessary to timely bring the silicone rubber composition into a predetermined cross-linked state when the organic foaming agent decomposes, in order to capture the generated gas.
In the cross-linking by peroxide, the cross-linking rate can be adjusted to a certain degree by selection of an organic peroxide and a compounding amount, but it is difficult to strictly adjust the cross-linking density at a timing of decomposition of the organic foaming agent, thus bringing about a disadvantage that it is required to compound an excessive amount of organic foaming agent or to strictly manage process conditions.
An addition-type thermosetting silicone rubber using a platinum catalyst (see Patent Reference 1 (JP-B 1972-043294 (KOKOKU))) or this silicone rubber in combination with the above-described cross-linking by peroxide (see Patent Reference 2 (JP-A 1977-081378 (KOKAI))) has a disadvantage that the rubber is inferior in preservation stability when the cross-linking density at decomposition of the foaming agent is optimized at a usual production rate and is inferior in productivity when priority is given to the preservation stability because a long time is required to bring the rubber into a predetermined cross-linked state, though the adjustment of the cross-linking density at the timing of decomposition of the foaming agent is relatively easy.
Patent Reference 3 (JP-A 2010-047646 (KOKAI)) and Patent Reference 4 (JP-A 2003-213132 (KOKAI)) disclose ultraviolet irradiation curable silicone compositions using a platinum complex activated with ultraviolet rays. However, the cured products obtained from the compositions disclosed therein are rubber or gel.